Immunoreactivity to antinucleoside antibodies is the basis of our rapid quantitative means to score DNA damage and repair in fixed animal cells. These (fluorescein or peroxidase labeled) antibodies are specific for denatured or single-stranded nucleic acids. Nuclear immunoreactivity in G-1 HeLa cells correlates well with DNA damage. In normal S-phase this is temporally related to DNA synthesis. Interference from RNA reactivity has not been a problem. Chemical carcinogens induce DNA damage (Exptl. Cell Res. 103: 175, 1976) which is readily detectable by our method. We are now developing methods for fixing animal cells in suspension, followed by staining with fluorescein labeled antinucleoside antibodies so that flow cytofluorometry may be applied. This application of the new technology has not been achieved before. It should be applicable to studies of freshly isolated human lymphocytes. The methods may provide a rapid novel means to test for carcinogen action on cells, including those from exposed humans. Diagnostic ultrasound induced immunoreactivityy in G-1 HeLa cells. Repair synthesis and cell transformation (C3H 10T-1/2 cells) was found after exposure to diagnostic ultrasound. This is under further study to learn if ultrasound has potential hazard for the humaan fetus. Protein synthesis inhibitors induce immunoreactivity in nuclei of G-1 HeLa cells. Unlike results with carcinogens, DNA breaks do not result and immunoreactivity induction is readily reversible upon removal of the inhibitor. The molecular basis of this is under study. It is an important exception to the apparent general correlation of immunoreactivity induction and mutagenicity. It may signal changes in the state of the chromatin in animal cells, i.e., uncovering of (immunoreactive) denatured regions of the chromatin normally shielded by rapidly turning over proteins.